1. Field of the Invention
This invention relates to a process for the production of fine-particle, solid, pourable or free-flowing useful materials or mixtures of such useful materials, which are suitable as and/or for use in wetting agents, detergents and/or cleaning preparations, from aqueous preparations thereof. The process according to the invention uses the known principles of spray-drying, except that superheated steam is now used as the hot gas stream.
The spray-drying of aqueous preparations of useful materials of the type mentioned has been carried out worldwide on an industrial scale for decades. Hot air or mixtures of air and hot waste combustion gases are used as the drying gas stream. Washing powders or useful materials and/or mixtures thereof for the production of laundry detergents in pourable and free-flowing powder form are industrially obtained in corresponding spray-drying towers, generally at ambient pressure, either in co-current or more frequently in countercurrent. From the extensive specialist literature available, reference is made purely by way of example to Masters, K. "Spray Drying--An Introduction to Principles of Operational Practice and Applications", Leonard Hill Books, London, An Intertext Publisher, 1972.
The advantages and disadvantages of this drying process using hot air as the drying gas are well known and are discussed in detail in the specialist literature. In addition to the wide availability of the gas phase, the advantages include inter alia the possibility of operating in open systems (normally corresponding spray-drying towers) which provide for easy disposal of the hot gas phase by discharge into the atmosphere. In addition, detailed physico-chemical studies of the drying process itself have shown that drying with hot air takes place effectively and quickly, even where comparatively mild hot gas temperatures are used. The drying process on the water-containing droplet of material begins at comparatively low temperatures, for example of the order of 40.degree. C., irrespective to a large extent of the temperature of the hot gas used, and continues effectively with a comparatively slow increase in the temperature of the droplet to the boiling range of water under normal pressure, so that the material to be dried is only subjected to a moderate heat effect. In overall terms, the drying process in hot air takes place quickly and effectively, even in its final phases, so that drying of the useful materials to the freeflowing product can be accompanied by a comparatively mild heat effect on the useful materials in the drying process.
However, the disadvantages and limitations of spray drying are also generally known, particularly in the field with which the present invention is concerned, namely the drying of useful materials or mixtures thereof, for example for laundry detergents and/or cleaning preparations. Reference is made purely by way of example to the following points: numerous useful materials in this field are sensitive to oxidation, particularly as organic components. The treatment with hot air can lead to substantial losses of value, particularly at relatively high temperatures. The drying of organic or substantially organic useful materials, for example corresponding surfactants based on natural materials, creates considerable problems arising out of the danger of fire or even explosion of the material being dried. Important components of the useful material, more particularly detergent-range nonionic surfactants, show a more or less pronounced tendency towards pluming and are discharged from the tower together with the steam-laden waste air. In overall terms, there is an increased danger of environmental pollution from the large throughput of water-containing, solid and gaseous materials or auxiliaries through the spray-drying tower. Attempts at recycling the drying gas stream have been largely unsuccessful in the industrial application of this process.
2. Discussion of Related Art
It has been known from the beginning of this century that superheated steam may be used instead of hot air for drying water-containing preparations of useful materials. Initial proposals along these lines go back to the year 1908. The possibilities of drying processes using super-heated steam as the hot gas medium have been closely investigated in the literature, particularly over the last few decades, and compared with the drying processes based on hot air typically encountered in practice. From the extensive literature available, reference is made to the following publications which, in turn, contain extensive literature indexes on this subject: A. M. Trommelen et al. "Evaporation and Drying of Drops in Superheated Vapors" AIChE Journal 16 (1970), 857-867; Colin Beeby et al. "STEAMDRYING", Soc. of Chem. Eng., Japan, Tokyo (1984), 51-68 and W. A. Stein "Berechnung der Verdampfung yon Flussigkeit aus feuchten Produkten im Spruhturm (Calculating the Evaporation of Liquid from Moist Products in Spray-Drying Towers)", Verfahrenstechnik 7 (1973), 262-267. From the more recent patent literature, reference is made to EP-A1 058 651 and EP-A2 153 704.
The drying processes using superheated steam as the hot gas stream, which are now carried out in practice, use comparatively uncomplicated wet materials as the material to be dried. Thus, corresponding industrial processes have been developed for drying wet lignite, sand, for the production of dry animal feeds or for drying paper pulp. All these materials may be regarded as comparatively unproblematical under the operating conditions required for drying, particularly in regard to time and temperature. One of the above-cited publications reports on detailed studies to extend this operating principle to the drying of wet materials of varying origin. In "Evaporation and Drying of Drops in Superheated Vapors" loc. cit., A. M. Trommelen et al. describe detailed studies of isolated drops of various wet materials having a predetermined size on the one hand during their treatment with flowing hot air and on the other hand with flowing hot steam. Studies were made inter alia on the temperature dependence of the drying rate, the characteristic temperature profile in the particular drop investigated in dependence upon the hot gas stream used and its predetermined temperature and characteristic deviations of the dried material particles to be compared with one another. The materials to be dried include pure water, various water-containing preparations from the food sector, such as aqueous sucrose solution, tomato juice, coffee extract and milk, solutions and suspensions of inorganic materials in the form of a clay suspension and aqueous solutions of sodium sulfate and potassium nitrate and, finally, drops of an aqueous solution of a commercially available laundry detergent. Particulars of the composition of the laundry detergent used are not provided. More particularly, studies were made into the effects of the working temperature of the particular hot gas stream on the drying process and the drying rate, the temperature profile within the drop of material to be dried over the duration of the drying process and into the effect of the particular solids content on the temperature profile of the drop of material during the drying process and, finally, the physical characteristics of the dried particle of material. Investigations into the chemical characteristics of the dried material and/or its reuseability were not conducted, particularly in the case of the laundry detergent. More particularly, no study was made of the extent to which temperature-sensitive mixture components are damaged during the complete drying of the multicomponent mixture.
The following observations may be made in respect of all the samples of material investigated:
At temperatures of 150.degree. C., the drying rate is distinctly higher for hot air than for the correspondingly super-heated steam. Only an increase in the operating temperature to 250.degree. C. brings the respective drying rates closer together, although even then hot air generally still shows some advantages. This result is in accordance with other works on the same subject which shows that it is only at around 400.degree. C. that the drying rates of the two hot gas media are entirely comparable with one another.
Another basic difference crucial to the understanding of the present invention is that, where the hot air stream is used, the drying process effectively begins at lower material temperatures. The elimination of water is so pronounced that, on reaching a material temperature of around 100.degree. C., the drying process is substantially complete. For example, up to about 90% of the total water present in the drop has been removed by that time. The temperature profile of the drop is totally different where superheated steam is used. Under the effect of the condensation of the superheated steam on the cooler starting material and the release of the heat of condensation to the material to be dried, the water-containing drop is spontaneously heated to the boiling temperature of the water under operating conditions, i.e. to temperatures of around 100.degree. C. where drying is carried out under normal pressure. This boiling temperature is maintained as the minimum temperature in the drop of material throughout the drying process. Depending on the degree of drying of the drop, the particular extent to which the aqueous phase is charged with the dry materials to be recovered leads to individual upward deviations in the temperature profile at an earlier or later stage.
Another known parameter of industrial drying processes is of considerable significance in conjunction with the problem addressed by the present invention, namely: in the spraying of solutions and/or slurries of useful materials from the field of wetting agents, detergents and/or cleaning products with which the invention is concerned, drops varying considerably in their individual particle sizes are formed and exposed to the drying process in the hot gas stream. This is reflected in the sieve analysis of the dry tower powder obtained, according to which most of the particles range, for example, from 0.05 to 0.5 mm in diameter. The actual particle size of the particular drop is a factor of considerable influence in respect of the individual drying state of the particular particle in question in the hot gas phase. The time required for complete drying of the particular drop increases excessively with increasing drop size so that, when individual drops of different particle size are compared with one another, the quotient of the particular time required to establish a predetermined degree of drying amounts to several times the corresponding ratio of the particle sizes to one another. For example, the relevant specialist literature mentions multiplication factors of the order of 20 to 100 for the particular limits in question.
Spray-drying processes are known to operate with a comparatively low solids density within the drying space, so that temperature equalization can only take place to a very limited extent in the partly or already extensively dried material through collision and contact of the individual particles with one another. Accordingly, the spray-drying zone differs clearly from heating in a fluidized bed, for example, in which the fine-particle material is used in a very much higher density, so that effective temperature equalization can take place between the individual particles.
Accordingly, the following conclusions may be drawn in regard to replacement of the hot gas stream based on air by superheated steam. The high evaporation rates necessary for operation on an industrial scale require comparatively high working temperatures where superheated steam is used. In the case of wet material, the temperature reaches the limit of 100.degree. C. from the outset and can rapidly move upwards in the final phase of the drying process under the effect of the predetermined high temperature of the superheated steam. The fine particles of material are thus particularly at risk in the final phases of the drying step in which the coarse particles of material are still not sufficiently dry.
For temperature-sensitive materials of the type with which the invention is concerned, this understandably involves considerable dangers. Something else which has to be taken into consideration is the fact that--well-known in the field of drying with superheated steam--that the dry material discharged from the process contains water vapor which immediately condenses during the cooling phase and, hence, remains in the material as residual moisture. The overall outcome of the difficulties--discussed only briefly herein--of using superheated steam as a drying medium for starting materials of the type targeted by the invention would appear to be that this known principle has never been applied in practice to the mixtures of materials to be used in accordance with the invention.
The problem addressed by the present invention was to establish conditions under which, despite the difficulties described above, hot air could be replaced by superheated steam, for example in the production of powder-form, storable, pourable and free-flowing laundry detergents or in the recovery of corresponding commercial forms of useful materials from this field, such as surfactants or surfactant-containing multicomponent mixtures.